This application is filed under 35 U.S.C. .sctn. 371 and based on PCT/EP98/00277, filed Jan. 20, 1998, now WO 98/32818.
1. Field of the Invention
This invention relates to a new type of cooling lubricant emulsion for the cutting of metals and to a process for the preparation of such an emulsion.
2. Discussion of Related Art
Cooling lubricants are preparations/mixtures which are used for cooling and lubricating the tools during metal cutting and metal forming. The most important processing operations are differentiated by the type of movements made by the processed part and the tool, by the geometry of the parts being produced and the processing conditions. One distinguishes, for example, milling, turning, drilling and grinding as being cutting processes, and rolling, deep drawing and cold extrusion as being deformations without cutting.
The common principle of the metal-cutting processes is that the cutting edge of the tool cuts into the material and in doing so removes a splinter from the surface, so that a new surface is formed. Very high pressures are required for cutting into the material. The deformation of the splinter and the friction produced under the pressure produce heat, which heats the workpiece, the tool and, above all, the splinters.
The required effect of using cooling lubricants is therefore the lowering of the temperature, which otherwise may rise, for example, up to 1000.degree. C. in the splinters and which affects the dimensional accuracy of the parts produced. Another major task of the cooling lubricant is to extend the useful life of the tools, which wear rapidly under the influence of elevated temperatures. The roughness of the surfaces is decreased by the use of a cooling lubricant, as the lubricant prevents welding of the tool and the surface of the workpiece and avoids adhesion of particles. Moreover, the cooling lubricant assumes the task of transporting away the splinters formed.
A clear definition of cooling lubricants has been established in the revised version of DIN 51385 No. 1, the items in question being cooling lubricants which are immiscible with water, water-miscible and mixed with water. According to DIN 51385, the term "mixed with water" refers to the fmal condition of the prepared medium (in most cases oil-in-water emulsions), but "water-miscible" refers to the condition of the concentrate.
Cooling lubricants mixed with water are prepared on the user's premises by mixing together a concentrate of the water-miscible cooling lubricant and tap water. Generally ca. 5% aqueous emulsions are prepared. The advantage of this type of cooling lubricant is the good cooling action, which is due to the thermal properties of the water. As a result of the good cooling action, it is possible to achieve very high operating speeds and thereby to increase the productivity of the machines. The lubricating action of the cooling lubricants mixed with water is adequate for most processing methods involving cutting. A further advantage lies in the low costs, which are achieved owing to the feasibility of mixing the concentrate with water. A disadvantage of cooling lubricants mixed with water is that they are susceptible to external influences, in particular to attack by microorganisms, and therefore require more control and care than do cooling lubricants which are immiscible with water, such as cutting oils, grinding oils and forming oils.
The Table below provides a summary of the requirements for cooling lubricants which are water-miscible and for those which are mixed with water:
cooling and lubricating action PA1 rust protection PA1 no attack on non-ferrous metals PA1 toxicological safety, in particular skin tolerance PA1 no foam formation PA1 no attack on paints and seals PA1 emulsion stability PA1 no agglutination or resinification PA1 good miscibility PA1 pleasant aroma PA1 clean appearance PA1 good filterability PA1 trouble-free disposal.
A survey of the processes for forming metals and of the auxiliaries conventionally used for this purpose may be found, for example, in Ullmann's Encyclopaedia of Industrial Chemistry, 5th Ed., Vol. A15, 479-486. The range of the available forms of the suitable auxiliaries extends from oils, via oil-in-water emulsions, to aqueous solutions.
Cooling lubricants which are immiscible with water and those which are water-miscible are frequently based on mineral oil. The grades of mineral oils used are predominantly combinations of paraffinic, naphthenic and aromatic hydrocarbon compounds. Besides the mineral oils, so-called synthetic lubricants ("synthetic oils"), such as polyalpha olefins, polyalkylene glycols and polyalkylene glycol ethers, dialkyl ethers, acetals, natural ester lubricants, as well as synthetic esters and derivatives thereof are also important.
To be capable of fulfilling the requirements in practice, cooling lubricants must contain various components in addition to the oil base. The most important groups of substances are the emulsifiers, anti-corrosion additives, biocides, EP additives, polar additives, antifogging additives, antioxidants, solid lubricating additives and defoaming agents.
Emulsifiers (for example, surfactants, petroleum sulfonates, alkali soaps, alkanolamine soaps) stabilise the fine distribution of oil droplets within the aqueous operating liquid, which is an oil-in-water emulsion. The emulsifiers are quantitatively an important group of additives for the water-miscible cooling lubricants.
Conventional anti-corrosion additives (for example, alkanolamines and salts thereof, sulfonates, organic boron compounds, fatty acid amides, aminodicarboxylic acids, phosphate esters, thiophosphonic esters, dialkyldithiophosphates, monoalkylaryl sulfonates and dialkylaryl sulfonates, benzotriazoles, polyisobutene succinic acid derivatives) are intended to prevent the rusting of metal surfaces. Some anti-corrosion additives simultaneously have emulsifying properties and are therefore also used as emulsifiers. Biocides (for example, phenol derivatives, formaldehyde derivatives, "Kathon MW") are intended to inhibit the growth of bacteria and fungi. EP additives (for example, sulfurised fats and oils, phosphorus-containing compounds, organochloro compounds) are intended to prevent microwelding between metal surfaces at high pressures and elevated temperatures. Polar additives (for example, natural fats and oils, synthetic esters) increase the lubricating properties.
Antioxidants (for example, organic sulfides, zinc dithiophosphates, aromatic amines) ensure that the cooling lubricants have a long-pot life.
In addition to the cooling action, the second important function of cooling lubricants is the lubricating action (see the article by W. Klose: "Kuhlschmiermittel auf Metalloberflachen", Mitteilungen des Vereins Deutscher Emailfachleute, 41, Number 11, pages 138-142 (1993)). According to this, the action of the lubricating components depends on the formation of surface layers which possess a lower shear strength than that of the underlying material and therefore reduce friction and wear. The range of surface conditions extends from adsorptively bonded layers, via chemisorption, to chemically reactive layers, which produce a strong bond to the metal surface.
Adsorptive lubricating films are the simplest form of lubricating covering on a surface. They are produced, for example, by mineral oils without specialised additives. The formation of the adsorbed layers may be promoted by additions of polar active substances, such as fatty alcohols or fatty esters. Here, over and above the purely physical adsorption, there occurs an interaction between the metal surface and the molecules of the lubricant, which results in a partial chemisorptive bonding of the fatty alcohol or of the fatty ester.
Fatty acids are typical examples of chemisorptive lubricating layer formers. The hydrophilic carboxyl group is chemically bonded to the metal surface by reaction with the metal atoms and the hydrophobic hydrocarbon group is aligned vertically to the surface. The increased adhesive strength of the chemisorptive layer improves the capacity to absorb pressure compared with that of purely adsorptive lubricating films, but, in many cases of metal forming, is still not sufficient to reduce friction and wear. Here only admixtures of EP or AW additives (extreme pressure or antiwear additives) bring about an adequate improvement in the lubricating performance, so that even complex forming processes are rendered possible. These additives are generally active substances containing chlorine, phosphorus or sulfur. The action thereof depends on the development of chemically reactive layers in the form of metal chlorides, metal phosphates or metal sulfides. For reasons of disposal, nowadays endeavours are made to dispense with chlorine-containing EP additives if possible. The reactive layers formed at the metal surface act, on the one hand, as solid lubricating films, which are constantly worn away and renewed during the forming process. On the other hand, they form monomolecular surface films, which may take up additional lubricating components.
Cooling lubricants mixed with water are a common type of cooling lubricant. In practice, however, different cooling lubricants mixed with water are used in order to satisfy the different requirements with regard to corrosion protection of the various materials processed, lubricating action at high operating speed, pot-life and, not least, industrial safety provisions and environmental requirements. Many different types of cooling lubricant concentrates have therefore to be produced, stored and transported in small batches by the manufacturers of these substances. At the user's premises, emulsions which are still usable may have to be discarded, if another type of cooling lubricant becomes necessary because of a change in materials. These processes are cost-intensive and disadvantageous from the environmental aspect.
There is accordingly a need for a new cooling lubricant emulsion of the type mixed with water, which may be used for an extended range of applications. Such a new type of cooling lubricant is made available through the finding that it is possible, by the application of high shear energy, to emulsify a natural cutting oil which is immiscible with water in a conventional per se cooling lubricant emulsion mixed with water, and thereby to obtain a stable oil-in-water emulsion. Such a combination of at least two oil components may be used for a wide range of applications.